1. Field of the Invention
The present invention relates generally to particle analyzing apparatuses and more particularly is concerned with a circuit arrangement incorporated into such analyzing apparatus for detecting abnormal functioning of the particle sensing function of the analyzing apparatus caused by some obstruction thereof.
2. Description of the Prior Art
In U.S. Pat. No. 3,259,891 to Coulter et al. there is disclosed a debris alarm for a particle analyzing apparatus, wherein the particle pulses are monitored by a higher than normal threshold. The portions of the pulses above this threshold are converted to constant amplitude pulses, which are integrated to obtain their widths. If a given pulse width exceeds some predetermined width, an alarm condition is triggered.
In particle sensing undertaken in the above described particle analyzer, which is sold under the trademark Coulter Counter.RTM., there are several possible situations where debris in the liquid sample suspension being processed can affect the particle count. First, debris that passes through the sensing aperture generally results in a long pulse, which is usually a result of the debris being long in length and/or proceeding through the aperture at a rate slower than smaller particles. This condition results in the loss of counts for particles during the time period in which the debris is passing through the orifice. Secondly, the debris can partially block the sensing aperture, resulting in particle pulses that have abnormally short widths and abnormally high amplitudes. Such a condition results in bad mean cell volume measurements, due to the debris caused reduction of the aperture's diameter and off axis longer trajectories of the particles through the aperture path. Moreover, in systems having positive displacement fluid movement through the aperture, a partial blockage will increase the velocity of the sample flow through the aperture, thereby narrowing the width and increasing the amplitude of the particle pulses. As a result, particle pulses can be generated that are too narrow for a proper response by an amplifier in the detection circuitry of the particle analyzer. Additionally, various sources of electronic noise can create narrow pulses that can be confused as particle pulses.
In view of the possible debris condition described above, it can be seen that U.S. Pat. No. 3,259,891 is only able to detect abnormally long pulses on a pulse-by-pulse basis. Hence, this design is deficient in the following ways: This design is not able to detect pulses that are abnormally short, such as those caused by electronic noise or by a partially blocked aperture. Secondly, the design of the prior art requires that the pulse exceed a relatively high threshold level, meaning that some debris may generate particle pulses, yet not exceed this threshold. Thirdly, the technique only examines pulses on an individual basis, and does not provide a benchmark or reference point for determining how much debris exists in the processed sample. In other words, a single piece of debris might trigger the alarm, and it might be the only piece of debris in the sample.
Accordingly, it can be seen that there is a need in the particle analyzer art to have a debris alarm which can detect pulses which are both too short and too long and, after such detection, determine how many such bad pulses there are with respect to some benchmark or reference number.